Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 SecurityLoan	Dec. 31, 2013 SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of loans	4,776	4,893
Pre-modification Outstanding Recorded Investment	$ 349	$ 443
Post-modification Outstanding Recorded Investment	270	338
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,391	3,749
Pre-modification Outstanding Recorded Investment	200	304
Post-modification Outstanding Recorded Investment	157	214
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,385	1,144
Pre-modification Outstanding Recorded Investment	149	139
Post-modification Outstanding Recorded Investment	113	124
Commercial, Financial and Agricultural [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	14	33
Pre-modification Outstanding Recorded Investment	25	72
Post-modification Outstanding Recorded Investment	23	34
Commercial, Financial and Agricultural [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	20	50
Pre-modification Outstanding Recorded Investment	6	7
Post-modification Outstanding Recorded Investment	3	3
Commercial Real Estate: Commercial Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	10	11
Pre-modification Outstanding Recorded Investment	38	41
Post-modification Outstanding Recorded Investment	13	14
Commercial Real Estate: Commercial Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1	4
Pre-modification Outstanding Recorded Investment	2	18
Post-modification Outstanding Recorded Investment	1	10
Commercial Real Estate: Construction [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1	6
Pre-modification Outstanding Recorded Investment	5	19
Post-modification Outstanding Recorded Investment		4
Commercial Real Estate: Construction [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans		1
Pre-modification Outstanding Recorded Investment		23
Post-modification Outstanding Recorded Investment		42
Commercial Real Estate Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	11	17
Pre-modification Outstanding Recorded Investment	43	60
Post-modification Outstanding Recorded Investment	13	18
Commercial Real Estate Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1	5
Pre-modification Outstanding Recorded Investment	2	41
Post-modification Outstanding Recorded Investment	1	52
Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	25	50
Pre-modification Outstanding Recorded Investment	68	132
Post-modification Outstanding Recorded Investment	36	52
Commercial Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	21	55
Pre-modification Outstanding Recorded Investment	8	48
Post-modification Outstanding Recorded Investment	4	55
Real Estate - Residential Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	453	676
Pre-modification Outstanding Recorded Investment	27	43
Post-modification Outstanding Recorded Investment	27	43
Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	381	119
Pre-modification Outstanding Recorded Investment	29	12
Post-modification Outstanding Recorded Investment	29	12
Home Equity: Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1,184	1,708
Pre-modification Outstanding Recorded Investment	79	91
Post-modification Outstanding Recorded Investment	72	86
Home Equity: Key Community Bank [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	674	161
Pre-modification Outstanding Recorded Investment	41	17
Post-modification Outstanding Recorded Investment	36	17
Home Equity: Other [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	158	227
Pre-modification Outstanding Recorded Investment	4	6
Post-modification Outstanding Recorded Investment	4	6
Home Equity: Other [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	310	212
Pre-modification Outstanding Recorded Investment	9	7
Post-modification Outstanding Recorded Investment	8	6
Home Equity Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1,342	1,935
Pre-modification Outstanding Recorded Investment	83	97
Post-modification Outstanding Recorded Investment	76	92
Home Equity Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	984	373
Pre-modification Outstanding Recorded Investment	50	24
Post-modification Outstanding Recorded Investment	44	23
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	37	49
Pre-modification Outstanding Recorded Investment	2	2
Post-modification Outstanding Recorded Investment	1	1
Consumer Other - Key Community Bank [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	45	31
Pre-modification Outstanding Recorded Investment	2	1
Post-modification Outstanding Recorded Investment	2	1
Consumer Credit Card Financing Receivable [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	290	629
Pre-modification Outstanding Recorded Investment	2	5
Post-modification Outstanding Recorded Investment	2	4
Consumer Credit Card Financing Receivable [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	514	240
Pre-modification Outstanding Recorded Investment	4	2
Post-modification Outstanding Recorded Investment	2	1
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	206	360
Pre-modification Outstanding Recorded Investment	17	24
Post-modification Outstanding Recorded Investment	14	21
Consumer Marine [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	373	272
Pre-modification Outstanding Recorded Investment	54	51
Post-modification Outstanding Recorded Investment	31	31
Other Consumer [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	38	50
Pre-modification Outstanding Recorded Investment	1	1
Post-modification Outstanding Recorded Investment	1	1
Other Consumer [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	67	54
Pre-modification Outstanding Recorded Investment	2	1
Post-modification Outstanding Recorded Investment	1	1
Consumer Other [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	244	410
Pre-modification Outstanding Recorded Investment	18	25
Post-modification Outstanding Recorded Investment	15	22
Consumer Other [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	440	326
Pre-modification Outstanding Recorded Investment	56	52
Post-modification Outstanding Recorded Investment	32	32
Consumer Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,366	3,699
Pre-modification Outstanding Recorded Investment	132	172
Post-modification Outstanding Recorded Investment	121	162
Consumer Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,364	1,089
Pre-modification Outstanding Recorded Investment	141	91
Post-modification Outstanding Recorded Investment	$ 109	$ 69